Luminescent phosphors (LP) have shown promising utility in biomedical imaging, cancer and antimicrobial therapies, telecommunications, lasers, display technology, water treatment and solar radiation capture. Rare-earth doped fluoride luminescent phosphors have been shown to vary greatly in their photoluminescence intensities depending on the crystal size, degree of crystallinity, and morphology. For NaYF4 and other iso-structural lanthanide-based materials, the hexagonal (β) phase has been shown to have relatively brighter photoluminescence relative to the cubic (α) phase.
Such fluoride-based luminescent phosphors have traditionally been synthesized under conditions that either are restrictive in scale, or lead to excessive particle growth. Template-based methods rely on the use of capping agents, such as oleic acid, to direct the structural formation of plate-like or rod-like crystalline particles. Recently, preparation of β-NaYF4 by a microwave-assisted route in ethylene glycol at 160° C. without the use of a template has been achieved. See, N. Niu, F. He, S. Gai et al, J. Mater. Chem. 22: 21613 (2012).
The use of reduced temperatures and template-free synthetic methods is desirable to improve scalability of fluoride-based luminescent phosphors. However, in known methods the resulting crystals are often greater than 1 micron which limits their application. There is a need for new methods that provide better control of the size of the crystals so as to provide luminescent phosphors with optimal or custom performance characteristics.